The ferrite, Alnico and rare-earth magnets have been used for various purposes, e.g., motors. However, these magnets are mainly produced by the sintering method, and have various disadvantages. For example, they are generally fragile and difficult to be formed into thin or complex-shape products. In addition, they are low in dimensional precision, because of significant shrinkage of 15 to 20% during the sintering step, and need post-treatment, e.g., grinding, to improve their precision.
On the other hand, bonded magnets have been recently developed, in order to solve these disadvantages and, at the same time, to develop new applications. Bonded magnets are generally produced by filling them with a magnet powder using a thermoplastic resin, e.g., polyamide or polyphenylene sulfide resin, as the binder.
Of these bonded magnets, those comprising iron-based magnet powder, especially the one containing a rare-earth element, tend to be rusted and lose the magnetic characteristics in a high temperature, humid atmosphere. To overcome these problems, the surface of the compact is coated with a film of, e.g., thermosetting resin, phosphate (as disclosed by Japanese Patent Laid-Open No.208321/2000), to prevent rusting. Nevertheless, however, they are still insufficient in rust-preventive effects and magnetic properties, e.g., coercive force.
It is necessary, when an iron-based magnet powder containing a rare-earth element is kneaded together with a resin for a bonded magnet, to crush the magnet alloy powder to several microns, in order to secure sufficient magnetic characteristics. The magnet alloy powder is normally crushed in an inert gas or solvent. However, finely crushing a magnet powder causes a problem. The finely crushed powder is so active that, when coming into contact with air before being coated, it will be rapidly rusted by oxidation to lose its magnetic characteristics.
Several attempts have been made to solve the above type of problems. For example, a magnet alloy powder is slowly oxidized, after it is crushed to several microns, with a very small quantity of oxygen introduced into the inert atmosphere. Another measure is coating the crushed magnet powder with a phosphate, as disclosed by Japanese Patent Laid-Open No.251124/1999.
However, the crushed magnetic particles agglomerate with each other by the magnetic force. Such a powder, although improved in resistance to weather in a dry atmosphere, is not satisfactorily improved in the practically important resistance in a humid atmosphere, even when the agglomerated particles are protected with the coating film, conceivably because of insufficient protection of the individual particles. Therefore, coating the powder still fails to solve the problem.
Under these circumstances, small-size motors, acoustic devices, office automation devices or the like have been recently required to be still smaller, which requires the bonded magnets therefor to have still improved magnetic characteristics. However, the magnetic characteristics of the bonded magnet of the conventional iron-based magnet powder containing a rare-earth element are insufficient for the above purposes. Therefore, it is strongly desired to improve magnetic characteristics of bonded magnets in the early stage by improving resistance of the iron-based magnet powder containing a rare-earth element to weather.
Another important problem to be solved is to increase energy product of the magnet itself. Energy product of a bonded magnet, which contains a resin, is naturally limited to a certain level. For a magnet to have an energy product higher than that of a bonded magnet, it is necessary to increase its apparent density to a level close to the intrinsic density of the magnet powder. One of the common methods therefor is sintering, described above. Another method is hot compression molding to compact the magnet powder. For example, a Nd—Fe—B-based magnet powder produced by the rapid quenching method can be formed into an isotropically compacted magnet having an energy product of 14MGOe at the highest, when hot-pressed. An Sm—Fe—N-based magnet powder is decomposed, when heated at 600° C. or higher, and several methods have been investigated to solve this problem, including hot isostatic pressing (HIP) (Powder and Powder Metallurgy, No. 47, 2000, pp. 801), impact compression (Japanese Patent Laid-Open No.77027/1994) and conductive powder rolling (Japanese Patent Laid-Open No.294415/2000). Nevertheless, however, none of these methods still give a compacted magnet of sufficient resistance to weather. The compacted magnet is also demanded to have improved weather resistance, as is the case with the above-described bonded magnet.
It is an object of the present invention to provide an iron-based magnet powder containing a rare-earth element, characterized by high resistance to weather and controlled deterioration of coercive force in a humid atmosphere, to solve the problems involved in the conventional techniques. It is another object to provide a resin composition containing the same powder for bonded magnets. It is still another object to provide the bonded magnet and compacted magnet produced by using the same powder.